VIENNA — Acting at Ukraine’s request, the United States and 14 other nations on Tuesday formed a military observer mission intended to monitor the tense Crimea region, and a U.S. official said the team planned to leave within 24 hours.

Daniel Baer, the chief U.S. delegate to the Organization for Security and Cooperation, told The Associated Press that each country was contributing two individuals. But with more OSCE member nations expected to join, he said the mission could grow beyond its present strength of 30 people.

The 57-nation OSCE works on consensus, so most monitoring missions would have to be approved by all nations, including OSCE member Russia. But a provision of its regulations allows member countries to ask others to send unarmed military monitors in case of emergencies, and Baer said that Ukraine made use of that rule.

OSCE officials responsible for minority rights, freedom of the media and representing the head of the organization already are on the ground in Ukraine, and Baer said they were en route to the Crimea on Tuesday night. A senior OSCE human rights official planned to join them Wednesday, he said.

But most of those officials are permanent appointees who regularly travel to OSCE countries. The military observation mission was being drawn up specifically for Ukraine. Baer said its weeklong mandate could be extended on Ukraine’s request. It also is alone in focusing precisely on the greatest immediate danger — the potential of military conflict between Russian and outgunned Ukrainian forces.

On Saturday, heavily armed pro-Russian forces took over the strategic Crimea Peninsula in Ukraine, surrounding its ferry, military bases and border posts. Two Ukrainian warships remained anchored in the Crimean port of Sevastopol, blocked from leaving by Russian ships.

Baer described the mission as a “broad-based monitoring effort” meant in part to prevent a possible “military incursion” and to give space for dialogue. He said the mission will observe “areas where there has been tension or uncertainty has arisen over lack of clarity over military movements.”

The OSCE includes Russia, the United States, all European countries and some central Asian ones.